Blender mirroring not working
You applied the modifier, that's why.

I only have a short time to explain, but I'll try:

Modifiers are non-destructive while they have been added to an object, but not yet applied. Once a modifier is applied, the effect is committed to the mesh permananetly. It's kind of like flattening layers in GIMP or Photoshop. It's a one-way street. 

There are times when you want to apply a modifier and times when you don't. If, for example, you had sculpted a symmetrical body using the Mirror Modifier, but then wanted to sculpt spikes on just one arm, obviously you would want to apply the modifier and then do the non-symmetrical parts.